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Why do farther away objects appear smaller

  1. Jul 7, 2015 #1
    Why do objects that are farther away appear smaller. The moon for example looks small from earth, but I don't understand why.
     
  2. jcsd
  3. Jul 7, 2015 #2

    russ_watters

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    It's geometry. The angle between two parts of an object gets smaller as the distance from them increases. You can draw a diagram.
     
  4. Jul 7, 2015 #3

    Nugatory

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    The apparent size of an object is (pretty much by definition) the angle that it subtends across our field of view. If you keep the size of the object constant while you increase the distance to the object, that angle gets smaller. Draw a few right triangles with the same length for one of the sides meeting and the right angle and different lengths for the other side meeting at the right angle, compare the angle opposite the fixed side, and you'll see this effect clearly.
     
  5. Jul 7, 2015 #4
    Or try this one:
    Hold your hand in front of your face so that nearly everything in the background is obscured by it.
    Most of your field of view sees your only your hand.
    Now move your hand away to arm length - Now you can see more of what is in the background right?
    Did your hand actually get smaller?, no, it just got further away.
     
    Last edited: Jul 7, 2015
  6. Jul 7, 2015 #5
    Scheuerf ; You asked: Why do objects that are farther away appear smaller. The moon for example looks small from earth, but I don't understand why.

    Reference https://www.physicsforums.com/threads/why-do-farther-away-objects-appear-smaller.822290/

    An object's apparent size{ visual angle in astronomy } is the angle it subtends at the eye. See Wikipedia, "visual angle" for drawing. Plain old geometry.The larger or closer the object, the bigger the angle and bigger the real though upside down image on the eye. Our brain's"computer" enhances and rights the image and ultimately uses past experience to give the perceived size. A close basketball appears bigger, a far basketball appears smaller. Our brain knows better most of the time. Perceived size is also affected by surrounding objects like the horizon.The brain is fooled. Notice how huge the moon or sun seems to get as it approaches the horizon. A rather striking effect.View the moon through a paper tube to eliminate peripheral cues and the size appears the same.
     
  7. Jul 8, 2015 #6
    Some say that objects, even people, actually shrink, the further they are. This is peripherally supported by the related theory: Out of Sight, Out of Mind.
     
    Last edited: Jul 8, 2015
  8. Aug 7, 2016 #7
    So if we had flat eyes and not lenses, then objects would appear to be the same size no matter ho far a way they were?
    Surely this is possible to emulate with a CCD that had no lens to concentrate the light ?
     
  9. Aug 7, 2016 #8

    A.T.

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    You wouldn't see any objects at all.
     
  10. Aug 7, 2016 #9
    You'd need telecentric lens for it: http://www.lhup.edu/~dsimanek/3d/telecent.htm
     
  11. Aug 7, 2016 #10

    russ_watters

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    A bare sensor with no lens doesn't focus light to create an image.
     
  12. Aug 16, 2016 #11
    The reason that objects become smaller with radial or linear distance is because we have two eyes and view objects from each eye creating a triangle from the object to the location of each eye. The brain coordinates both views into a single image. The coordination of seperate views creates as triangle that will show the exact rate of visual size reduction based on the distance from the object to the point where both eyes see the same object in perfect focus or it disappears from visible sight. This a simple but accurate explanation. I hope this helpful!
     
  13. Aug 16, 2016 #12
    Having two eyes and all that is related to the way our brain can estimate distance to objects and give us the sensation of 3D vision.

    However, the size of distant objects is smaller even in images given by a single "eye" systems, like a photographic camera.
    Similarly, the image of distant objects on the retina is indeed smaller and fewer sensor cells are excited. The triangle to be considered for this is the one with the point in the eye and the base on the objects. The angle at the eye is smaller as the (same) object gets farther away.
    This angle can be increased by optical system like tele-objectives or telescopes and then we feel that the object looks larger.
     
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